Electrostatic spray finishing is a painting process that uses the particle-attracting properties of electrostatic charges to gain peak efficiency in spray operations. Electrical charges are generally applied to the paint particles in one of two ways: by induction charging or by ion bombardment.
Both types of electrical charging take place during the process of atomization. Induction charging occurs when the paint is still in contact with the high-voltage electrode or metal injector in the tip of the nozzle of the spray gun. Ion bombardment, on the other hand, alters the paint droplets as they are forced out through the gun's nozzle because of the ionization of air around the electrode of the metal injector tip.
Generally, in electrostatic spray operations, a negative charge is applied to the coating, while the target product is grounded. When the electrostatically charged paint droplets are introduced into an electric field of the grounded target, they behave like tiny magnets, and the lines of force in the field (i.e. the corona) become the lines along which the paint particles are carried to the target. As the level of d-c voltage increases, so does the density of the lines of force. Consequently, the higher the voltage, the greater the number of lines of paint that will wrap around the edges of the target, thus enhancing the coating application at the edges.
The coating-to-product transfer efficiency of electrostatic spray finishing operations is high because as the paint particles are attracted to the target they literally wrap around it. This principle, commonly referred to as wrap-around, is one of the primary reasons this finishing technique can result in 60-90% transfer efficiency in coating material usage when compared to other finishing operations in which a great deal of paint is lost to overspray or blow-by. This marked savings in material is a prime motivating factor in the movement toward electrostatic finishing techniques.
The use of electrostatics, like any other technology, has its limitations. The electrostatic attraction of any coating material is greater on outer edges and hole edges, thus causing a heavier buildup in these areas. This edge phenomenon is caused by magnetic forces that are concentrated on an object's outer surfaces, and any sharp edge becomes a collection point. However, this excess buildup can be controlled by the application method and the applied charge.
Another problem associated with electrostatic spray finishing is what is known as the Faraday cage effect, caused by the focused concentration of the applied charge. As a result, only minimal amounts of coating reach recessed areas of the target, especially on parts with complicated configurations. If both the target and paint are charged with opposite polarity, however, the Faraday cage effect is dramatically reduced, virtually eliminating the need for additional touch-ups. Still, in some cases, a separate conventional air spray application is advised to ensure complete coverage.
There are two basic types of electrostatic spraying systems currently in use. The first is known as an uninhibited or nonresistive system. Uninhibited systems are characterized by the application of voltage or electron flow directly to the atomization device. The coating material is fed through the atomizer where it picks up a high-voltage charge. There is very little resistance, if any, placed in the atomizer cable, the power supply or the atomizer. An uninhibited system requires stringent control measures because the entire atomizer is charged. For this reason, uninhibited systems are usually incorporated only into automatic finishing operations in which it is possible to isolate the spray area.
Inhibited or resistive systems, on the other hand, involve the closely controlled channeling of high voltage through the spraying device by limiting the amount of current at the device. The power supply, in effect, pumps the electrons through a series of current-limiting resistors to the electrode. The wire is covered with insulating material which, in turn, is covered by a ground-shield. This precaution prevents the escape of voltage or current. The spray gun itself is made of an insulative material, and resistors within it control the flow of electrons as they make their way to the tip of the electrode and into the atmosphere, charging the paint particles.
In general, the inhibited system consists of features that allow the operator to handle the atomizing equipment as though it were any other electrical appliance. Handheld electrostatic guns are referred to as inhibited when the applied voltage ranges up to 90,000 V. Inhibited automatic electrostatic systems have voltage ranges up to 135,000 V.
In any system, whether uninhibited or inhibited, the atomization of the coating material and the velocity of the atomized particles are the major parameters for judging the efficiency of a system. Smaller particles are lighter and thus are more easily drawn to the grounded target object. The velocity of the coating particles should be fairly slow in order to avoid blow-by which occurs when the material moves past the target too fast to adhere properly. The greatest efficiency is usually achieved in systems offering optimum atomization coupled with the lowest possible velocity of particles.
In one system the force of air is utilized to atomize the coating particles. A high-voltage charge is induced into the spray pattern and electrostatically charges the atomized coating particles. The attraction between these charged particles and the object to be coated is powerful enough to cause the overspray to bend or wrap around the back side of the object. Electrostatic air spray systems normally offer good wrap-around performance, highly uniform film build and smooth finish, material savings, and reduced emissions. Most air spray equipment is adjustable, and liquid flow rates can be set up to 50 oz. per minute.
For example, the U.S. Patent to Watanabe et al U.S. Pat. No. 3,093,309 discloses electrostatic coating apparatus of the spray gun type which utilizes compressed air and a plurality of nozzles mounted on a single air spray gun to lower spray velocity. The lower air stream velocity increases the effect of the adsorbent force of the electric field applied to the atomized coating material. In this way, the volume of coating material flying out of the electric field is decreased. However, the plurality of nozzles mounted on a single air spray gun is awkward to move and control in order to uniformly coat a work surface. Each nozzle delivers a relatively small amount of atomized spray since the fluid flows under a relatively low fluid pressure.
Atomization can be accomplished through a number of other methods, including the use of rotary atomizers such as stationary and hand-operated bells. In a bell or disk system, centrifugal forces atomize the paint, and the high-voltage differential between the paint dispenser and the grounded target attracts the paint to the part.
Airless electrostatic spray systems use hydraulic pressure to atomize the fluid by discharging it through a small opening at pressures of 500-4500 psi. As the fluid is released, it is atomized into fine particles at a velocity sufficient to carry the atomized coating to the target. Airless electrostatic equipment is often used when overspray must be kept to a minimum and film buildup of three of four mils is required. Such systems are also used where fine finishes are not required.
A relatively new method used for electrostatic spray finishing is the air-assisted airless system. It also offers the wrap-around performance of other electrostatic equipment. In this system, medium fluid pressure--300-1000 psi--is used to atomize the coating material and shape it into the desired fan pattern. An air-assist is applied to the spray pattern, enhancing the atomization process and doing away with tails that would mar the finish.
The various spray devices may be fitted to handheld, automatic, or robot spray equipment. Handheld electrostatic spray gun systems usually consist of a handgun, fluid and air hoses, high-voltage cables, and a high-voltage power pack that converts a-c line voltage to d-c voltage. The power pack also contains air and electrically operated switches necessary to control air flow and electric voltage and current to the spray gun.
In manual spraying, all variables of the system, such as fluid flow rate, atomizing pressure, fan shape, and the sweep pattern of the gun, are selected by the operator. The operator must be skilled enough to detect when the film buildup is too heavy or not heavy enough. Manual spray finishing is most often used when a wide variety of parts must be painted.
Automatic systems incorporate some of the same components as those of manual systems: a power pack, high-voltage cables, and electrostatic spray guns. The high-voltage power pack, though, is usually wall-mounted and remote-controlled. The power pack used in automatic spray operations may often generate twice the electrostatic charge of handheld equipment, and this charge can be adjusted in relation to the type of coating material used. The high voltage cables must be insulated and capable of withstanding the powerful charge. The spray guns themselves may be fixed to the floor, on reciprocating arms, or in an overhead configuration. System variables are preset for the production run, and an operator monitors the system to ensure that these parameters remain within the specified tolerances. Although, in theory, it is possible to program the equipment so that all parts moving through the system receive an acceptable coating, some areas tend to receive a too-thick or too-thin application. Therefore, some degree of secondary touch-up is often warranted. Automatic systems are used when a limited number of different systems are used when a limited number of different parts are to be painted. High-production finishing of similar parts is almost aways carried out in an automatic system. An early example of such an automatic system is disclosed in the U.S. Patent to Tilney et al U.S. Pat. No. 3,279,421.
Robotic spray finishing systems are like standard automatic systems except that the spraying is performed by robots capable of mimicking the movements of a human painter. The robot is programmed to carry out the required tasks, and the program's speed is adjusted in relation to the speed of a conveyor as it moves the target parts through the spray area. Robots are currently being used in spray operations that are monotonous and repetitive, those that require complete and uniform application of the coating material, and those that pose serious health hazards to human operators.
Finishing robots have found a secure niche in the automotive industry. Automobile body contours are well-suited to electrostatic finishing, and the high production runs justify the cost of robotic systems. Examples of such robotic spray finishing systems which utilize rotary atomizers are disclosed in U.S. Patents to Vecellio U.S. Pat. Nos. 4,532,148 and 4,539,932 and the U.S. Pat. No. 4,601,921 to Lee Each of the Vecellio patents discusses the relatively low transfer efficiency of robotized air-spray gun systems which is attributed in large part to the use of high pressure air for atomizing the liquid coating material.